In situ retorting of oil shale by transient state fluid flows



Jan. 9, 1968 M. L. s uss ET AL 3,362,471

IN SITU RETORTING OF OIL SHALE BY I TRANSIENT STATE FLUID FLOWS FiledSept. 8, 1965 2 Sheets-Sheet 1 STEADY STATE FLOW I TRANSIENT STATE FLOWMARION L. SLUSSER DEAN K. WA LTON INVENTORS ATTORNEY Jan. 9, 1968 FiledSept. 8, 1965 COMPONENTS, PER CENT M. L. SLUSSER E Al. 3,362,471 IN SITURETORTING OF OIL SHALE BY TRANSIENT STATE FLUID FLOWS 2 Sheets-Sheet 2FIG.4

E23 OXYGEN [:1 CARBON DIOXIDE HYDROCARBONS I I I I I I I I 8 IO I2 14 I6I8 20 22 24 CYCLING TIME, HOURS FIG.5

[m OXYGEN III] CARBON DIOXIDE HYDROCARBONS I00 200 300 400 500 600 700800 900 I000 I230 PRESSURE LEVEL, PSIG MARION L. SLUSSER DEAN K. WALTONINVENTORS ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE Thisspecification discloses: A method for recovering hydrocarbons whereintransient state fluid flows are used to improve the in situ retorting ofthe oil shale in subterranean formations. More particularly, heating gasis injected into the oil shale to effect combustion of carbonaceousmaterials therein to generate retorting temperatures. The pressure ofthe heating gas is continuously increased to maintain transient fluidflowing conditions in the oil shale. Then, the pressurized fluids areremoved from the oil shale under continuously decreasing pressures tomaintain transient fluid flowing conditions therein. Hydrocarbon-s arerecovered from these fluids. The conditions controlling the injectionand removal of the above fluids may be varied, and the resulting changesmonitored, to obtain increased recoveries of hydrocarbons.

This invention relates to a method for recovering hydrocarbons from oilshale by in situ retorting. More particularly, it relates to the use oftransient state fluid flows for improving the in situ retorting of oilshale in subterranean formations.

Vast oil shale formations residing in many parts of the world are atremendous source of hydrocarbons. The oil shale formations arecomprised of a solid rock, which has an extremely low naturalpermeability to gases in the range of about 10- millidarcies or less,interspersed with natural fractures. The fractures permit fluid flowsthrough the oil shale to provide it with a much higher apparentpermeability to fluids. Thus, the oil shale without these fractureswould be impermeable to fluid flows.

Producing hydrocarbons from oil shale in place requires the applicationof a conversion process, which includes both heat transfer into and masstransfer out of the oil shale, to effect in situ retorting. The oilshale needs to be heated to above about 500 F. in order to convert thehigh molecular weight and solid organic material, kerogen, into fluidhydrocarbons. Fluid flows, to remove this finely dispersed fluid fromthe rock matrix, then are required for its final recovery. Heat transferinto oil shale is severely restricted by its low thermal conductivityand the initial extremely low natural permeability to fluids of thesolid oil shale. Further, heat and mass transfer into the oil shale arerestricted by the connterflow of the retorted fluids by diffusioninduced by concentration and vapor pressure gradients. The movement offluid is especially restrained by the very low porosity, free voidspace, of the oil shale; and outside of fractures, generally, theporosity is less than 1 percent.

After the oil shale is retorted free of fluidizable hydrocarbons, thereremains a coke residue. The permeability to fluids of the solid retortedoil shale with this coke residue is about 1 millidarcy. Burning thiscoke residue from this oil shale will increase the permeability tofluids to between 1 and 10 millidarcies. If burning is practiced in thesolid oil shale to an extent where inorganic carbonates are decomposed,the permeability to fluids can exceed 100 millidarcies. Thus, it will beseen that fluid movements through the oil shale, outside of fractures,will be heavily restricted under normal conditions even where it has itsgreatest permeability, as for example, through decomposition ofinorganic carbonates. If it is considered that the surfaces in the oilshale presented even to the fractures are relatively small when comparedto the amount of organic matter desired to be converted thermally intohydrocarbons, some means must be provided to increase heat and masstransfer conditions in the oil shale.

For this purpose to effect surface-retorting, the solid oil shale iscrushed to particles ranging in sizes from fine powder to several incheson a side. Thus, 40 percent, or more void space will exist in a bed ofcrushed oil shale. Now the small particles are free to expand and permita ready outflow of the produced hydrocarbons across small zones of spentshale as retorting is carried out on the crushed oil shale. All of thesefactors make for comparatively favorable conditions for the conversionand production retorting process. However, in a retorting process inmassive subterranean oil shale formations faced with the inherentproperties of the oil shale without the oil shale being crushed to smallsized particles, both heat transfer and mass transfer must occur acrossan ever-expanding thickness of spent shale. The spent shale has heattransfer properties similar to an insulating fire brick with equallyunfavorable flow properties. Since the oil shale has less than 1 percentporosity and practically no free void space, little or no movement ispossible Without lifting the overburden strata. These factors all makediflicult the effective carrying out of in situ retorting procedures forrecovering hydrocarbons from a subterranean oil shale formation, andgreatly delay the economic benefits of such process.

It is therefore an object of the present invention to provide a methodfor the in situ retorting of oil shale where the problems relating tothe low porosity, low permeability to fluids, small heat transfer, andrestricted mass transfer conditions of the oil shale are largelyovercome. Another object is to provide an in situ retorting methodwherein the oil shale is heated effectively, predominantly throughconvection, and in less time than by heretofore known procedures.Another object is to provide an in situ retorting method for recoveringhydrocarbons from oil shale in which fluid flows are controlled toincrease greatly the heat transfer and mass transfer conditions in theoil shale. Another object is to provide for in situ retorting of oilshale by steps which obtained optimum fluid flowing conditions withoutthe difficulties presented in heretofore known shale oil recoverymethods.

These and other objects will become more apparent when read inconjunction with the following detailed description, the appendedclaims, and the attached drawings, wherein:

FIGURE 1 shows a subterranean formation of oil shale, in verticalsection, provided with suitable apparatus for carrying out variousembodiments of the method of the present invention;

FIGURES 2 and 3 show the oil shale, of FIGURE 1, in fragmented verticalsections to illustrate a comparison between the in situ retorting of oilshale with fluids flowing in steady state flow and in transient stateflow, respectively; and

FIGURES 4 and 5 are charts illustrating the effect of cycling rates, andpressure levels, on transient state fluid flowing conditions in theretorting of oil shale by this invention.

The objects of the present invention are achieved by utilizing transientstate fluid flowing conditions by a plurality of novel steps in the insitu retorting of oil shale. These steps provide for retorting the oilshale and for recovering the produced hydrocarbons with more effectiveutilization of heat energy and greater amounts of recoveredhydrocarbons, all in less time, than heretofore u? believed obtainable.Certain variations may be practiced for improved recovery ofhydrocarbons, and for the optimization of the various steps, so that amaximum recovery of hydrocarbons may be obtained.

Referring to FIGURE 1 of the drawings, there is shown an oil shale 11provided with suitable apparatus for carrying out the steps of thepresent invention. The oil shale may be in any formation, as forexample, in the Green River Formation of Colorado. This oil shale 11contains vast quantities of hydrocarbons in the form of kerogen andcontains as high as 35 weight percent of kerogen content. The oil shale11 is usually interspersed with a plurality of fractures, although insome cases it may be void of natural fractures. The oil shale 11 has alow natural permeability to fluids of about millidarcies or less, with aporosity of less than 1 percent outside of any fractures. For practicingthe present method, the oil shale 11 is preferred to be withoutinterconnecting fractures through which fluids are unrecoverably lost.However, the fractures may be sealed with mud, grout, cement, and thelike, so that fluids flow only within a restricted area to be retorted.

The oil shale 11 is provided a well means 12 which serves as fluidcommunication between the earths surface and the interior portions ofthe oil shale 11. The well means 12 may comprise a wellbore 13 extendedinto the oil shale 11 by any suitable means. The wellbore 13 will beusually formed by rotary drilling. Disposed within the welbore i3 is acasing 14 which extends downwardly from the earths surface to near thebottom of the wellbore 13. The casing 14 is secured, by suitable means,in fluid-tight relationship with the oil shale 11 and usually with theoverburden 16. Preferably, the casing 14 is cemented to the overburden16 and the oil shale 11 which it contacts. Carried at the top of thecasing 14 is a wellhead 17 through which various conductors extend. Aconduit 18, for removing pressurized fluids from the oil shale 11,passes through the wellhead 17 and extends downwardly into the lowerextremities of the wellbore 13. The conduit l8 carries a valve 19 forregulating the flow of fluids therethrough and connects to a hydrocarbonrecovery system where fluids are processed to recover hydrocarbons. Thehydrocarbon recovery system is not of a critical nature to this methodand, thus, may be of conventional design. Many systems are known, andfor this reason, a system is not shown in FIGURE 1. Preferably, thevalve 19 is a motor valve controlled by signals supplied from a remotepoint.

Also, the wellhead 17 carries a conduit 21 connecting a source ofheating gas with the oil shale 11. The conduit 21 extends downwardlyinto the wellbore 13 with its lower terminus provided with a pluralityof openings 22 through which heating gas may be dispersed within thewellbore 13. A source of pressurized heating gas is provided and may beof any form, such as a compressor 23, or the like. The compressor 23, atits exhaust, connects to a conduit 24 carrying a control valve 25 and byinterconnection with a conduit 27 carrying a valve 28 to the conduit 21.Conduit 27 is connected with a pressure indicator 29 which may be apressure gauge or a differential manometer. The compressor 23 is drivenby a suitable prime mover for compressing the heating gas to elevatedpressures. The heatin gas then passes through the interconnectingconduits 24 and 27 to the conduit 21 for injection through the openings22 and the wellbore 13 into the oil shale 11. The pressure at which theheating gas is passed by the wellbore 13 into the oil shale 11 ismonitored by the pressure indicator 29. By suitable mechanisms, thepressure at which the heating gas flows can be adjusted automaticallythrough operable interconnection between the pressure indicator 20 andvalve 26. A similar system may be used for automatic control of fluidflow in the conduit 18 by interconnection with valve 19. Chainlinesindicate such interconnections. However, manual control of these valves19 and 26 may be practiced. The

valve 28 may be used in conjunction with the valve 26 to regulate theflow of the heating gas, if desired.

The oil shale 11 about the wellbore 13 may be heated by auxiliary heatsources to temperatures suflicient to effect ignition. For this purpose,an electrical heater 31 can extend to the bottom of the wellbore 13. Theupper extremity of the heater 31 passes through the wellhead 17 withsuitable electrical interconnection to a source of electrical power.Electrical power may be provided by an alternator 33 driven by primemover 34 of suitable design. The oil shale 11 surrounding the wellbore13 may be heated to ignition temperatures by means other than the heater31, if desired.

The described apparatus are operated in general for injecting a heatinggas into the wellbore 13 in conjunction with heating of the adjacent oilshale 11 to ignition temperatures whereby in situ combustion is effectedof carbonaceous materials therein. The valve 19 is closed at this time.The oil shale 11 is continued to be heated to temperatures of about 500F. or higher, and after the oil shale 11 surrounding the wellbore 13 isin part retorted, the injection of heating gas is terminated through theclosing of valve 26. Now the valve 19 is opened to convey thepressurized fluids from the oil shale 11 through the wellbore 13outwardly to the hydrocarbon recovery system connected thereto. With thevalve 28 open, the pressure at which the heating gas is injected, andthe fluids are removed, from the oil shale 11 via the wellbore 13 ismonitored by the pressure indicator 29. Thus, the well may be cycledbetween the injection of heating gas, to effect heating by in situcombustion for retorting the oil shale, and for depressuring thewellbore 13 to effect removal of the pressurized fluids from the oilshale 11 with recovery of the hydrocarbons that they contain. The oilshale 11 and the related apparatus which have been described foreffecting this procedure are considered to be conventional and variousother arrangements may be utilized, including more than one wellborespaced in fluid interconnection within the oil shale 11.

The term heating gas as used herein is intended to include a gasselected from the group consisting of oxygen-containing gases singularlyor in various proportions and combinations with combustible andnoncombustible gases. The heating gas may be preheated initially at thesurface by suitable equipment to a temperature sufficient to ignitespontaneously a portion of the carbonaceous materials in the oil shale11. The combustion of such materials produces heat to decompose thekerogen into hydrocarbons and to move the resultant hydrocarbons in theoil shale 11. The heating begins with the oil shale surfaces adjacentthe wellbore 13 and produces a combustion front which migrates outwardlythrough the oil shale 11. The combustion front moves progressively,radially outwardly from the well means depending upon the heat transferconditions in the oil shale 11.

The temperature and the velocity of the resulting combustion front canbe regulated by adjusting the tempera ture, volume, and pressure of theheating gas being introduced, or its oxygen content. However, since airis the most inexpensive source of heating gas, the only considerationsare temperature, pressure, and volume of the heating gas. The oil shale11 is preferably heated to a temperature above 500 F., and preferablybetween 500 F. and 1100 F. Temperatures below 500 F. are ineffectivecommercially to decompose the kerogen portion of the oil shale 11 intoshale oil products.

Combustible gases may also be injected into the oil shale 11 and thereinignited when the hydrocarbon content of the oil shale is relatively low,when initiation of combustion is difficult, or for other reasons. Suchcombustible gases can provide the only fuel used for heating the shaleor to supplement the carbonaceous materials or kerogens present asnatural fuel in such oil shale 11. The combustible gases may be injectedinto the oil shale 11 through the well means 12 separately or incombination with oxygen-containing gases. It may be found desirable incertain aspects of operation to inject alternately the oxygen-containinggases and the combustible gases into the oil shale l1. Noncombustiblegases may be admixed with the injected gases to assist in controllingthe temperature of the combustion front, if desired. The particularcomposition of the heating gas is not critical in this invention.

Prior to describing the present method, an orienting description of theflow states possible during the in situ retorting of oil shale isbelieved helpful. The flow of fluids in priorly known in situ retortingmethods has been in the steady state. In the steady state, a pressureedifferential exists between spaced locations in the oil shale.

11 which does not change significantly in magnitude with time, althoughthe pressures creating this differential at these locations may beslowly changing. Referring now to FIGURE 2, a portion of the oil shale11 receives a flow of the injected heating gas through a fracture 36 toeffect in situ retorting with fluid flows in the steady state. Thefracture 36 is assumed to be in fluid communication with the wellbore 13and an external vent with the heating gas flowing at a constant pressurefrom left to right as illustrated by the arrows. Combustion is effectedalong the surfaces 37 of the oil shale 11 along the fracture 36. Thezones 38, of the oil shale 11, are burned clean of carbonaceousmaterial. In zones 39 remains a carbonaceous residue after heating hasdecomposed kerogen into fluid hydrocarbons. The heating gas enters thefracture 36 at a constant pressure P The pressure P downstream in thefracture 36 is reduced by the naturally occurring pressure drop. At alocation in the oil shale 11 normal to the fracture 36, there will be atequilibrium established a pressure P in the clean zones 38. The pressuredifference between pressures P and P is slight, being created by theconcentration gradient of materials flowing countercurrently to theheating gas between these two pressure-locations and the thermallyincreased vapor pressure of the retorted products at P so that thepressures P and P are substantially the same for practical purposes. Inthe coked zones 39, the fluid hydrocarbons, from conversion of thekerogen, cannot move at the pressure P toward the near equal pressure PAlso, these fluids would have to move countercurrently to the flow ofheating gas. For this reason, these fluid hydrocarbons in coked zone 3-9at the location of the pressure P must move angularly downstream towarda point of reduced pressure P Since the difference between pressures Pand P is determined by pressure drop in the open fracture 36, and thefluids flowing in the zone 39 are subject to much greater restriction,then pressure P must be located downstream of pressure P to establishfluid flows thereto from the location of P into the fracture 36. Asubstantial flow path must then exist, in steady state flow, to movehydrocarbons from the location of pressure P into the fracture 36. Theproducts of the retorting and combustion reactions are moved from theinterior of the oil shale 11 into the fracture 36 to mix with the flowof the heating gas. These fluids are removed from the fracture 36 andprocessed to recover the hydrocarbons.

As a result of steady fluid flow, the heating gas only has a very lowdiffusion, or forced, flow perpendicularly into the oil shale 11 inducedby concentrationpressure gradients so that great amounts of time arerequired to produce a given quantity of hydrocarbons, and also with alow oxygen utilization. The hydrocarbons, from the conversion ofkerogen, must move through the oil shale 11 nearly parallel to thecombustion front existing in the zones 39 whereby large quantities ofthese hydrocarbons are consumed in this front and in the excess freeoxygen traversing the fracture 36. Thus, it is seen that the steadystate flow of fluids employed for retorting the oil shale 11 must resultin a relatively low production of hydrocarbons even from those smallamounts converted from the kerogen. Also, this reduced recovery 6 ofhydrocarbons from the oil shale 11 takes great lengths of time becauseof restricted fluids flow. A large amount of carbon residue in the cokedzones 39 remains out of contact with the oxygen needed to consume it.The basic problems appear to reside in heat transfer obtained primarilyby conduction and with mass transfer through only diffusion (molecular)flow in long flow paths in the steady state flow of fluids employed forin situ retorting.

The present method of in situ retorting of oil shale 11 is carried outemploying fluids in a transient state of flow. A great multiplication inheat and mass transfer quantums is Obtained principally by forced fluidflows. There is also a corresponding greatly increased production andrecovery of hydrocarbons in less time than such quantity of hydrocarbonscan be obtained with steady state fluid flows. The term transient stateflow of fluids is herein employed to define the flow of fluids inresponse to an increasing pressure differential existing between spacedpoints in the oil shale, and the pressures at those points creating thedifferential pressure are constantly changing unidirectionally inmagnitudes with time. With reference to FIGURE 3, the oil shale 11 has afracture 46 with one end sealed and the other in fluid communication tothe well bore 13. The fracture 46 receives, at a constantunidirectionally changing pressure, heating gas in a flow from right toleft in the direction of the arrows. The oil shale 11 adjacent thefracture 46 is heated sufficiently to effect ignition whereby an in situcombustion front results along surfaces 47. The zones 48 of the oilshale 11 are burned clean of organic material by the advance of suchfront. Zones 49 are heated by the front sufficiently to convert thekerogen into fluid hydrocarbons with only a carbonaceous coke remainingas residue. This coke is later consumed by the in situ combustion frontmoving normally and outwardly into the oil shale 11 to provide heat forretorting the virgin oil shale 11.

The heating gas passed into the fracture 46 at everchanging pressuresmaintains a transient state condition of fluid flow in the oil shale 11.Thus, the pressures of the heating gas at any location in the oil shale11 will be ever changing in magnitude with time. More particularly,pressures P and P at spaced locations on a line normal to the fracture46, are always of different magnitudes and changing at different rates.The pressure P always changes at a greater rate than pressure P so thata pressure differential increasing in magnitude with time is establishedthroughout the transient state flow of fluids in the fracture 46. Anatural pressure drop downstream from the location of pressure P willproduce pressure P in the fracture 46. There will be a differential inpressure between pressure P and pressure R; at a location on a linenormal to the fracture 46 in the oil shale 11. The differential betweenthe pressures P and P will be substantially the same in magnitude asbetween pressures P and P although there is also a differential inpressure between P and P When the pressure of the heating gas within thefracture 46 is increasing, the pressure differentials also increasebetween P P and P P to maintain continuously the flow of fluidsoutwardly in the oil shale 11 for effecting in situ retorting. When thepressurized fluids in the fracture 46 are removed at ever-decreasingpressure, the pressure differentials also increase but the directionalflow of fluids is reversed. However, the continuous fluid flows from theoil shale 11 into the fracture 46 are again obtained. Thus, in eitherevent, the increasing pressure differentials between the pressures P Pand P P and also between P and P continuously induced flow of fluidsbetween the locations of the pressures P and P and likewise between Pand P into the fracture 46 and then out of the oil shale 11. Fluids donot flow appreciably from the location of pressure P through the oilshale 11 to pressure P in the fracture 46 since the differential betweenpressures P P is obviously greater than between pressures P P As aresult, heat and mass transfers are accelerated by a continuous movementof fluids substantially perpendicularly between the oil shale 11 and thefracture 46. Thus, in situ retorting is principally obtained byconvection in the fluids moved perpendicularly between the fracture 46and the interior of the oil shale 11. This results in a greatlyincreased heating of the oil shale 11 by the continuous penetration ofthe heating gas over the shortest flow path from the fracture 46, and oflike advantage, in the rapid and thorough removal of the hydrocarbonsproduced by conversion of the kerogen into the fracture 46 and the wellbore 13 to facilitate recovery of the hydrocarbons. In summary,transient state fluid flowsmaintain pressure gradients increasingcontinuously within the oil shale 11 to stimulate greatly by convectionthe heat transfer and mass transfer required to retort effectively theoil shale 11 in situ.

A laboratory experiment and mathematical model study were carried out todetermine the effective differences between transient state and steadystate flow regimes illustrated in FIGURES 3 and 2, respectively. Fromthis work, it can be shown in applying fluid flows in the transientstate, a 2 /2 foot thick zone of depleted oil shale (like coked zone 4%)from a fracture can be flushed at a closed boundary between normalizedpressure levels of .98 and .65 in .55 hour for the depressuring toatmospheric from a given maximum pressure. For the same mass flow at thegiven maximum pressure, but under steady state flow conditions, the timerequired is roughly 13.8 hours. Also, data from the in situ retorting ofsamples of oil shale has shown, under similar comparable pressure andmass flow rates, that the steady state flow illustrated in FIG- URE 2produces a flow path between pressure locations P and P twelve times aslong as the flow path between the locations of P and P under thetransient state fluid flow illustrated in FIGURE 3. Thus, a very obviousgreat improvement is obtained in heat transfer, and mass transfer,conditions by the employment for in situ retorting of the transientstate flow in accordance with this invention rather than the steadystate flow of fluids of prior known procedures.

With the foregoing understanding of the basic premises of steady stateflow versus transient state flow, a preferred embodiment of theinvention will be described now with reference to FIGURE 1. As a firststep of this method, heating gas is injected for a first period of timethrough the well means 12 into the oil shale 11 from the conduit 21. Theheating gas, supplied by the compressor 23, is regulated by the valve 26for injection into the oil shale 11. Conjunctively, the oil shale 11adjacent the wellbore 13 is heated to ignition temperatures to effect insitu combustion of the resident carbonaceous materials. For thispurpose, the heating gas may be preheated at the earths surface of theheater 31 may be operated on energy supplied by the current source 33driven by the prime mover 34. After in situ combustion is obtained, theinjection of the heating gas is continued at ever-increasing pressuresto maintain transient state fluid flowing conditions in the oil shale11. For this purpose, the valve 26 may be adjusted manually. Preferably,the valve 26 is automatically controlled from pressure indicator 29which monitors the pressure at which heating gas is injected into theoil shale 11. Means other than valve 26 may be used to arrange theinjection of the heating gas into the oil shale 11 at ever-increasingpressures, if desired. The rate of increasing the pressures of injectionof the heating gas between upper and lower pressure limits may becontingent upon a time basis which insures the continuous diffusion ofthe various fluids by transient state flow into the oil shale 11.However, the injection pressures should be ever increasing so thattransient state fluid flowing conditions are maintained in the oil shale11. The injection of the heating gas under these conditions is continueduntil elevated pressures are obtained. The injection of the heating gasinto the oil shale l1 ends at a maximum pressure below the magnitudewhich induces fracturing in the 8 oil shale 11. At this time, the valve26 in the conduit 27 is closed. Obviously, it is undesirable to continueinjecting the heating gas while propagating fractures into the oil shale11 since the products of the in situ retorting would be lost toextremities of the oil shale 11 from where they cannot be recovered.

Another step of the method is practiced after the injection of theheating gas is ended. The pressurized fluids contained in the oil shale11 resultant from the preceding steps are removed under transient stateflow through the conduit 18 from the well means 12 with regulation offluid flow by the valve 19. The pressurized fluids from the conduit 18are applied to a hydrocarbon recovery system in which the hydrocarbonsretorted from the oil shale 11 are recovered. The valve 19 is employedto regulate the flow through the condit 18 of fluids removed fromwithinthe oil shale 11 at ever-decreasing pressures to maintaintransient fluid flowing conditions in the oil shale 11. The valve 19 canbe adjusted manually. Preferably, the valve 19 is controlledautomatically by arrangement with the pressure indicator 29, which withvalve 28 open, reflects the pressure of fluids entering the wellbore 13from the oil shale 11. The rate of decreasing pressures may be of thesame magnitude as described for the injection of the heating gas intothe oil shale 11. However, it is desired that the pressure decrease at asuflicient rate that the liquid hydrocarbons in the pressurized fluidsin contact with the heated oil shale 11 do not undergo unacceptablethermal degradation into gases.

Usually, the duration of the first and second periods of time (in whichthe injection of heating gas and the removal of the resultantpressurized fluids from the oil shale 11 are obtained) is extendedperiodically as the retorting progresses through the oil shale 11. Thisis of advantage since the flow path normal to the fracture 46 in the oilshale 11 between the location of pressures P and P increases as the insitu retorting proceeds. Consequently, both heat transfer and masstransfer by fluid flow, and conduction, are maintained relativelyconstant in rate across an ever-expanding thickness of the clean zones48. The spent oil shale in zones 48 has a heat transfer property similarto an insulating fire brick with fluid-flow properties equallyunfavorable. A greater oxygen utilization and greater recovery of theresultant hydrocarbons in the pressurized fluids are obtained byperiodically extending the duration of said first and second periods.

It is also preferred as retorting continues, for the same reasons asdescribed for extending the duration of the first and second periods,that the maximum pressure level of the injected heating gas in thefirst-mentioned step is periodically increased as retorting progressesthrough the oil shale. By increasing the pressure, a greater masstransfer by fluid flows is obtained through the areas of the clean zones48. Thus, the differential between P and P may be increased to fostergreater mass transfer and heat transfer conditions.

It has been found that an improvement in the porosity of the oil shalesubject to in situ combustion can be correlated with the steps of thepresent method so that even greater mass transfer and heat transferrelationship can be established in the employment of the transient statefluid flows as heretofore described. More particularly, with referenceto FIGURE 3, the oil shale 11 between fractures, prior to retorting,usually has less than 1 percent porosity although the coked zones 49have an improved porosity from the degradation of the kerogen intohydrocarbons. Further, the mean pore size in the coked zones 49 for a25-35 gallon per ton oil shale is found by measurement to have adimension of about 0.1 micron, which varies slightly but proportionallywith its kerogen content. In the clean zones 48 freed of coke,particularly when subjected to temperatures suitable to decompose thecarbonate constituents, the average pore sizes are much greater than the0.1 micron dimension in the coked zones 49. Thus, one major limitingfactor in the retorting of the oil shale 11 is the small average poresize in the coked zones 49.

Laboratory results have shown, with reference to FIG- URE 3, that thetotal flow of the gases under transient state flow conditions throughthe clean zone 48 and the coke zone 49 into contact with the oil shale11 is in the Knudsen regime where the mean free path of the heating gasis larger than the average pore diameter of the coke zone 49. However,the practical operational pressure levels, through which increasingpressures and decreasing pressures of the step of the present inventionare unidirectionally changed, may be sufliciently increased until themean free path of the heating gas becomes smaller than the averagediameter of the pores in the coke zones 49 and then the fluid flows arein the Poiseuille regime.

In both the Knudsen and the Poiseuille regimes, the total fluid flowwill include diffusional flow resulting from concentration gradients andthe forced flow resulting from the pressure gradients developed by thetwo mechanisms: (a) pyrolysis reactions resulting in large volumeincreases as the kerogen is converted to liquid and gaseous products;and (b) the pressure drop imposed on the fluids flowing through thefracture 46 under pressure differentials. Thus, in both flow regimes,the total volumetric transport depends on the pressure or concentrationgradient existing across the pore.

In the Knudsen regime where diffusional, or molecular, flow predominatesover forced flow, the mass rate of flow is proportional to theconcentration or partial pressure gradient between spaced locations foreach fiuidized component; and the constant of proportionality is termedthe diffusion coefiicient. This coefiicient varies inversely with theabsolute pressure in the system and therefore attains its highest valueat low pressures, i.e., subatmospheric. While this coeflicient is highat low-pressures, the expelling force from concentration (or partialpressure) gradients between spaced locations is small so that the totalmass transport is low.

However, at elevated pressures where flow is in the Poiseuille regime,the forced flow predominates over diffusional flow and the mass flowrate is proportional to the pressure gradient with the constant ofproportionality decreasing as the absolute pressure increases. Thisconstant is similar to the permeability coeflicient in Darcys equation.

The significance of the statements regarding the Knudsen and Poiseuilleregimes becomes more apparent when considered with the followingequations which define these regimes.

In the Knudsen regime, the rate of heating gas flow through a singlepore maybe given by the following Equation 1:

7r? Ag) dt 3 {m Ax (1) where:

dn/at is the flow in number of molecules per second r is the radius ofthe pore M is the molecular weight R is the gas constant T is theabsolute temperature Ap/ Ax is the partial pressure gradient along thepore (or Ac/Ax where using the concentration gradient).

3-5 wzTMRT M where x is the mean free path of the gas and the otherconstants are as defined for Equation 1. In the above regime, the totalmass transport through a pore is dependent upon the pressure orconcentration gradient existing across the pore in this pressure rangebut the mass transport through the pore is markedly increased byincreases in pressure gradient across the pore.

The mean free path A of a gas molecule is related generally to pressurep, by the Equation 3:

li cm 2 In practice, the flow in both regimes described by Equations 1and 2 can be combined into Equation 4:

ms 520L 11 dt 3V2 Aa: 64A (4) The terms are as defined for the previousequations.

At all pressures at which the mean free path of the gas is larger thanthe mean diameter of the pores in the coked zone 49, the flow of fluidsis within the Poiseuille regime and the rate of heating gas flow, orother pressurized gas flow, is described by Equation 1.

For all pressures at which the mean free path of the gas is smaller thanthe mean diameter of the pores in the coke zone 49, the fl-ow of fluidsis within the Poiseuille regime and the rate of heating gas flow, orother pressurized gas flow, is described by Equation 2.

It has been found, as previously explained, that the mean averagediameter of pores in the coke zones 49 of the oil shale 11 is about 0.1micron. Increasing the pressure level from 1 to 10 atmospheres across apore of this size causes the heating gas flow rate (corrected tostandard conditions) to increase through the pore approximately 18-fold.This result can be calculated from the foregoing equations and has beensubstantiated by laboratory experiments and model studies. The sameemphasized result of mass flow is found relative to the clean zones 48.If Darcys law is used for comparative proof, assuming the permeabilityconstant to be the same at both pressures, the volumetric rate of flowwould increase about 33-fold. In one laboratory experiment, thevolumetric rate of air flow (corrected to standard conditions) is shownto increase 26-fold by increasing the pressure level from 1 to 10atmospheres across the pores in coked zones 49. The calculatedpermeability constant decreased from about 3.3 millidarcies to anasymptotic value of 1.5 millidarcies over the same range of pressures.Thus, operating transient flows of fluids in the oil shale 11 atpressures where the mean free path of the gas phase is less than themean pore diameter of the coked zone 49 provides about a 20-foldincrease therethrough in the volumetric flow rate of fluids. Thisflowrate improvement increases all of the desirable factors relating tothe retorting and recovering of hydrocarbons from oil shale 11.Particularly, oxygen utilization is greatly increased, and due to themass and heat transfer increases which result, greater portions of theoil shale 11 can be retorted in less time, and much greater amounts ofhydrocarbon can be recovered, than could be obtained by transient flowconditions at pressures where the fluid flow is defined by the Knudsenregime, i.e., where the mean free path of the gas phase is larger thanthe mean average pore diameter of the coke zones 49.

The steps of the present method in a preferred aspect include theinjection of the heating gas, under transient fluid flowing conditions,at elevated pressures where the injected gas molecules have a free pathless than the average pore-radius of the porous matrix of the oil shale11 subjected to in situ combustion, particularly in the coke zones 49.Thus, there will be a great increase in the volumetric rate of heatinggas flow with all the aforementioned advantages in the retorting of theoil shale 11. It has been found that these advantages can be obtained inshale in the Green River Formation by increasing the pressure levels inthe injection of heating gas at transient flow conditions to above 10atmospheres. Inasmuch as the oil shale 11 to be retorted usually residesat relatively great depths below the surface of the earth, pressuresextending significantly above this 10-atmosphere level can be usedwithout inducing fracturing in the oil shale 11.

The same advantageous results may be obtained in the step wherepressurized fluids are removed from the oil shale 11 through the wellmeans 12. Generaly, the pressure levels in this step will be arranged toconform with those of the preceding step so that the injection ofheating gas terminates at the level where removing of pressurized fluidsbegins, and begins at the pressure where removal of pressurized fluidsfrom the oil shale 11 ends so that a true cyclic procedure may bepracticed with great facility.

It is of special utility in the steps of injecting the heating gas andremoving the pressurized fluids from the oil shale 11 that the injectionof heating gas begin at a first pressure of suflicient magnitude wherethe injected gas molecules have a free path about equal to the averagepore radius of the porous matrix of the oil shale 1 1 subjected to insitu combustion, particularly the coke zones 49; and that the injectionend at an elevated pressure greater in magnitude than the firstpressure, but not of suflicient magnitude to induce fracturing in theoil shale 11. Thus, the entire injection of the heating gas underconditions effecting a transient state in the flow of fluids in the oilshale 11 takes place in a range of pressures in the Poiseuille regime sothat the greatly multiple increase in the volume-rate of heating gasflow through the pores of the oil shale 11 is obtained. Likewise, theremoving of the pressurized fluid from the oil shale 11 begins at aboutthe second pressure where the injection of the heating gas wasterminated, and ends when the pressurized fluids in the oil shale 11 areabout the first pressure where the injection of heating gas began. Thus,a cyclic program for effecting greatly increased recovery ofhydrocarbons from oil shale with a corresponding decrease in the timerequired to obtain such recovered hydrocarbons will be effected.

It has been found in the present method that by certain improved stepseven greater improvements in the recovery of hydrocarbons from the oilshale 11 can be obstreams, particularly the volume and composition ofthe pressurized fluids removed from the oil shale, adjustments can bemade to the basic steps of the present invention for increasing therecovery of hydrocarbons from the pressured fluids. Further, by makingcertain adjustments to the conditions under which these steps arepracticed, a maximum recovery of hydrocarbons can be obtained.

It has been found that the duration of the first and second periods,where injection of heating gas and the removal of pressurized fluids areobtained, the maximum pressure level to which the heating gas isinjected, and the difference between this maximum pressure and the leastpressure to which the pressurized fluids are removed, is adjusted,either singularly or conjunctively, so as to increase the recovery ofhydrocarbons from the pressurized fluids. Further, making particularadjustment to each of these conditions for practicing the basic steps ofthis invention provides a maximized recovery of hydrocarbons from thepressurized fluids.

In example, the step of monitoring the volume and composition of thepresurized fluids removed from the oil shale 11 through the wellbore 13is practiced. The apparatus, and oil shale structure in which it isemployed, may be substantially as shown in FIGURE 1 for this purpose.The step of monitoring the volume and composition of the pressurizedfluids provides information from which adjustment may be made to thesteps of this procedure for increasing the recovery of hydrocarbons fromthe pressurized fluids.

The steps of the present invention were practiced in a field test, alongwith monitoring of the volume and composition of the pressurized fluidsremoved from the oil shale 11, in the Green River Formation in Colorado.Particularly, the steps of the present invention were practiced with themaximum pressure levels of the injected heating gas successivelyincreased in 100 p.s.i.g. increments from 100 p.s.i.g. to 500 p.s.i.g.in cycles from atmospheric pressure. The duration of the first andsecond periods was held substantially constant with a variation onlybetween a 4- and 6-hour duration The bottom-hole temperatures were alsomonitored. The composition and volume of the pressurized fluids removedin the conduit 18 were monitored. The information obtained is reportedin the following Table I: 1

1 Maximum injection pressure above atmospheric pressure. 2 Averagebottom-hole temperature.

tained. To facilitate practicing such improved steps, in reference toFIGURE 1, means are provided to monitor the volume and the compositionof pressurized fluids removed from the oil shale 11 through the conduit18. Any means may be used to measure the volume of pressurized fluid inthe conduit 18. Gas analysis, performed by any suitable means on thepressurized fluids, may show the hydrocarbon, oxygen, carbon dioxide,carbon monoxide, and hydrogen contents of the pressurized fluids. If desired, bottom-hole temperatures within the wellbore 13 may also betaken. The amount of oxygen and related gases contained in the heatinggas injected through the conduit 21 into the oil shale 11 may bemonitored so that oxygen utilization and hydrocarbon production can beobtained by comparison of the analysis made on the flow of fluids in theconduits 21 and 18. It has been found that from the information obtainedby analysis of these fluid In Table I, it will be observed that bycycling of the injected heating gas/air from atmospheric pressure to apressure level maximum of 400 pounds and removal of the pressurizedfluids down to atmospheric pressure, a 5- hour duration for the firstand second periods of time produced a maximum hydrocarbon recovery andwith a maximum consumption of oxygen in the heating gas. The maximumpressure levels, above and below this 400- pound pressure, producelesser amounts of hydrocarbons in the same amount of time. From thisinformation in Table I, it is of great facility to adjust the basicsteps of this invention for a pressure cycle between atmospheric and 400p.s.i.g. from the information obtained by monitoring the volume andcomposition of the pressure fluids so as to increase the recovery ofhydrocarbons from the pressurized fluids. For example, if the steps ofthe present invention were operated at 300 p.s.i.g. maximum pres- 13sure level for the injecting heating gas, then the pressure level may bereadily increased to 400 p.s.i.g. for the same duration of the first andsecond periods of time to obtain about a 30 percent increase inhydrocarbon recovery from the pressurized fluids removed from the oilshale 11 through the conduit 18.

Likewise, other information obtained from monitoring the volume andcomposition of the pressurized fluids can be used to advantage. There isshown graphically in FIGURE 4 additional information derived by cyclingthe well means 12 between atmospheric and 100 p.s.i.g. maximum pressurelevels for the injected heating gas and removed pressurized fluids fordurations of the first and second periods of A, 2, 6, and 24 hours. Theinformation, displayed in bar-graph style, indicates that about the6-hour duration for the first and second periods produces a maximumhydrocarbon production and also a maximum oxygen utilization, a minimumoxygen content, and a maximum carbon dioxide content, in the pressurizedfluids removed from the oil shale 11. Thus, the information obtainedfrom this step may be used for varying the conditions in the rate ofpressure cycling in the steps of this procedure to improve greatly therecovery of hydrocarbons.

Again, by monitoring the volume and composition of the pressurizedfluids removed from the oil shale 11, and from the information obtained,it is possible to adjust the pressure diflterential which producestransient fluid flowing conditions, during the steps of injecting theheating gas and removing the pressurized fluid, to improve hydrocarbonrecovery. In FIGURE 5, a graphic display of the information thusobtained shows the effect of magnitude changes in differential pressurebetween minimum and maximum pressures on composition of the producedpressurized fluids. For this purpose, the well means 12 were employed inthe steps of the method with the heating gas injected from atmosphericto several diiferent maximum pressure levels. These pressuredifferentials ranged from 100 p.s.i.g. to 1230 p.s.i.g. as displayed inFIGURE 5. The oxygen, carbon dioxide and hydrocarbons contents areillustrated in bar-graph styles. From FIGURE 5, clearly a pressurechange from atmospheric to 400 p.s.i.g. levels for the injected heatinggas and removed pressurized fluids provided maximum hydrocarbonrecovery. Similarly, the oxygen utilization for the injected heating gas(air) was the greatest as shown by a minimum residual oxygen content anda maximum carbon dioxide production. The magnitude of the pressuredifferential producing these results is dependent upon the range ofpressures through which the change of. pressure is effected. However,the range of pressures over which transient-state fluid flows are madecan be adjusted to obtain an increased recovery of hydrocarbons by thesesteps for any magnitude of pressures.

The step of monitoring the volume and composition of the pressurizedfluids removed from the oil shale provides information on which theconditions of practicing the basic steps of this invention are varied,one at any time, until an increased recovery of hydrocarbons may beobtained. Then, after adjustment to these steps as required to increasethe hydrocarbon recovery and information determined by the precedingsteps, the steps again can be repeated until a substantial quanity ofhydrocarbons is recovered from the shale. More particularly, through theinformation obtained in the described manner, the duration of the firstand second periods of time, during which the injection of heating gasand removal of pressurized fluids occur, respectively, is adjusted toproduce an increased recovery of hydrocarbons from the pressurizedfluids. Similarly, the maximum pressure level to which the heating gasis injected and pressurized fluids removed is adjusted to produce anincreased recovery of hydrocarbons from the pressurized fluids.Likewise, the pressure differential between maximum injection pressureand minimum removal pressure which produce transient fluid flowingconditions in the oil shale 11 is adjusted to produce an increasedrecovery of hydrocarbons from the pressurized fluids.

Varying each of the conditions under which the basic steps are practicedand then repeating the step adjusted from information obtained willproduce a maximum recovery of hydrocarbons. More particularly, each ofthe conditions under which the basic steps are practiced is varied withmonitoring of the volume and composition of the pressurized fluidsremoved from the oil shale 11. During this time, the duration of thefirst and second periods of time during which the heating gas isinjected and the removal of pressurized fluids occurs, respectively, isadjusted to produce an improved hydrocarbon recovery; the maximumpressure level to which the heating gas is injected into the oil shale11 is adjusted to produce an increased recovery of hydrocarbons; thepressure differential producing transient fluid flowing conditions inthe oil shale 11 during the injection of heating gas into and theremoval of pressurized fluids from the oil shale 11 is adjusted toproduce an increased recovery of hydrocarbons; and these steps practicedconjunctively provide a maximum of recovery of hydrocarbons in retortingin situ the oil shale 11. The variation and adjustments of these stepscan be practiced in any order to the same effect without loss ofadvantage.

It has been found that, at some time during the practice of these steps,it may be desired to increase further the permeability of the cleanzones 48 in the oil shale 11. For this purpose, the maximum pressurelevel to which the heating gas is injected, and the duration of thefirst and second periods of injecting the heating gas and removing thepressurized fluids, are adjusted so the bottom-hole temperature withinthe wellbore 13 increases sufliciently to decompose the carbonateconstituents in the oil shale 11. This results in producing largeconcentrations of carbon dioxide beyond that which is indicativenormally of the consumption of the oxygen in the heating gas. Thus, thepermeability of the clean zones 48 may be greatly increased to fostergreater increases in mass and heat transfers therethrough from thefluids flowing between the fracture 46 and the interior of the oil shale11. Also, practicing these steps at certain elevated temperatureconditions influences the range of hydrocarbon products produced bypyrolysis of the kerogen. Thus, the conditions for these steps may bevaried and adjusted so as to provide a maximum, or an increased,recovery of a desired, or optimum, range of hydrocarbon products.

No exact mathematical criterion or relationship between the variousconditions under which the steps of this invention are practiced can beestablished at this time. For this reason, it is necessary to rely uponintentional variation in the conditions under which these steps arepracticed to obtained the information which allows adjusting the stepconditions for the increased recoveries of hydrocarbons from thepressurized fluids and, where desired, to increase to a maximum therecovery of hydrocarbons thus produced.

It will be apparent from the foregoing that there has been hereindescribed a method employing the transient flows of fluids in theinjection of heating gas and the removal of pressurized fluids from oilshale to produce a greatly increased recovery of hydrocarbons by in situretorting than heretofore could be obtained and in a much reduced periodof time. Various changes and alterations to the steps of this inventionmay be made without departing from its scope, and it is intended thatsuch changes and variations be encompassed within the scope of theappended claims. For this reason, it is intended that the presentdescription of this invention is to be taken as illustrative and notlimitative for the purposes of definition in the present claims.

What is claimed is:

1. A method for the in situ retorting of oil shale mass iii having wellmeans in fluid communication therewith, comprising the steps:

(a) injecting for a first period of time a heating gas from the wellmeans into the mass of oil shale with heating of said oil shale to atemperature sufficient to effect combustion of the carbonaceousmaterials contained therein, said injection carried out at everincreasing pressures to maintain transient fluid flowing conditions inthe oil shale, ending said injection of heating gas at a maximumpressure less than sufficient in magnitude to induce fracturing in theoil shale, and thereafter,

(b) removing by the well means for a second period of time the resultantpressurized fluids from the oil shale, which received the heating gas,at ever decreasing pressures to maintain transient fluid flowingconditions in the oil shale, and recovering hydrocarbons from thepressurized fluids removed from the oil shale.

2. The method of claim 1 wherein the duration of the first and secondperiods is extended periodically as retorting progresses through the oilshale.

3. The method of claim 1 wherein the maximum pressure level of theinjected heating gas is periodically increased as retorting progressesthrough the oil shale.

4. A method for the in situ retorting of oil shale mass having wellmeans in fluid communication therewith, comprising the steps:

(a) injecting for a first period of time a heating gas from the wellmeans into the mass of oil shale with heating of said oil shale to atemperature suflicient to effect combustion of the carbonaceousmaterials contained therein, said injection being carried out at everincreasing pressures to obtain transient fluid flowing conditions in theoil shale with a substantial portion of said injection of heating gasbeing made at pressures where the injected gas molecules have a freepath less than the average pore radius of the porous matrix of the oilshale after subjected to in situ combustion, ending said injection ofheating gas at a pressure of less than suflicient in magnitude to inducefracturing in the oil shale, and then,

(b) removing by the well means for a second period of time pressurizedfluids from the oil shale which received the heating gas at everdecreasing pressure to maintain transient fluid flowing conditions inthe oil shale, and recovering hydrocarbons from the pressurized fluidsremoved from the oil shale.

5. A method for the in situ retorting of oil shale mass having wellmeans in fluid communication therewith, comprising the steps:

(a) injecting for a first period of time a heating gas from the wellmeans into the mass of oil shale under in situ combustion conditions atpressures increasing continuously to maintain transient state fluidflowing conditions for establishing a pressure gradient in the oil shalematrix adequate to effect heating of same to at least retortingtemperatures, said injection beginning at a first pressure of asuflicient magnitude where the injected gas molecules have a free pathabout equal to the average pore radius of the porous matrix of the oilshale after subjected to in situ combustion and said injection ending ata second pressure greater in magnitude than the first pressure and yetnot of sufficient magnitude to induce fracturing in the oil shale, andthen,

(b) removing by the well means for a second period of time pressurizedfluids from the oil shale which received the heating gas at continuouslydecreasing pressures to maintain transient state fluid flowing conditions beginning at about the second pressure and ending when thepressurized fluids in the oil shale are at about the first pressure, andrecovering hydrocar- 13 bons from the pressurized fluids removed fromthe oil shale.

6. A method for the in situ retorting of oil shale mass having wellmeans in fluid communication therewith, comprising the steps:

(a) injecting for a first period of time a heating gas from the wellmeans into the mass of oil shale with heating of said oil shale to atemperature sufficient to effect combustion of the carbonaceousmaterials contained therein, said injection carried out at everincreasing pressures to obtain transient fluid flowing conditions in theoil shale, ending said injection of heating gas at a pressure of lessthan suflicient in magnitude to induce fracturing in the oil shale,

(b) removing by the well means for a second period of time pressurizedfluids from the oil shale which received the heating gas at everdecreasing pressure to maintain transient fluid flowing conditions inthe oil shale, and recovering hydrocarbons from the pressurized fluidsremoved from the oil shale,

(c) monitoring the volume and composition of the pressurized fluidsremoved from the oil shale and on the basis of the information thusobtained,

(d) making adjustment to the steps (a) and (b) as required forincreasing the recovery of hydrocarbons from the pressurized fluids, andthen,

(e) repeating the adjusted steps (a) and (b) until hydrocarbons areremoved in a substantial quantity from the oil shale.

7. A method for the in situ retorting of oil shale mass having wellmeans in fluid communication therewith, comprising the steps:

(a) injecting for a first period of time a heating gas from the wellmeans into the mass of oil shale with heating of said oil shale to atemperature suflicient to effect combustion of the carbonaceousmaterials contained therein, said injection carried out at everincreasing pressures to obtain transient fluid flowing conditions in theoil shale, ending said injection of heating gas at a pressure of lessthan sufficient in magnitude to induce fracturing in the oil shale,

(b) removing by the well means for a second period of time pressurizedfluids from the oil shale which received the heating gas at everdecreasing pressure to maintain transient fluid flowing conditions inthe oil shale, and recovering hydrocarbons from the pressurized fluidsremoved from the oil shale,

(c) monitoring the volume and composition of the pressurized fluidsremoved from the oil shale while varying one of the conditions at anytime under which the steps (a) and (b) are practiced until an increasedrecovery of hydrocarbons is obtained,

(d) repeating the steps (a) and (b) with the adjustment to these stepsrequired to increase the recovery of hydrocarbons as determined in step(c), and then,

e) repeating the adjusted steps (a) and (b) until a substantial quantityof hydrocarbons is recovered from the oil shale.

8. The method of claim 7 where in step (c) the duration of the first andsecond periods of time, during which the injection of heating gas andremoval of the pressurized fluids occur, respectively, is adjusted topro-' duce an increased recovery of hydrocarbons from the pressurizedfluids.

9. The method of claim 7 where in step (c) the maximum pressure level towhich the heating gas is injected into the oil shale is adjusted toproduce an increased recovery of hydrocarbons from the pressurizedfluids.

10. The method of claim 7 where in step (c) the pressure differentialproducing transient fluid flowing conditions in the oil shale during theinjection of heating gas into, and the removal of pressurized fluidsfrom, the oil shale is adjusted to produce an increased recovery ofhydrocarbons from the pressurized fluids.

11. A method for the in situ retorting of oil shale mass having wellmeans in fluid communication therewith, comprising the steps:

(a) injecting for a first period of time a heating gas from the wellmeans into the mass of oil shale with heating of said oil shale to atemperature suificient 5 to effect combustion of the carbonaceousmaterials contained therein, said injection carried out at everincreasing pressures to maintain transient fluid flowing conditions inthe oil shale, ending said injection of heating gas at a pressure ofless than suflicient in magnitude to induce fracturing in the oil shale,

(b) removing by the well means for a second period of time pressurizedfluids from the oil shale which received the heating gas at everdecreasing pressure to maintain transient fluid flowing conditions inthe oil shale, and recovering hydrocarbons from the pressurized fluidsremoved from the oil shale,

(c) monitoring the volume and composition of the pressurized fluidsremoved from the oil shale while varying each of the followingconditions under which the steps (a) and (b) are practiced until amaximum recovery of hydrocarbons is obtained:

( 1) the duration of the first and second periods of time during whichthe injection of heating gas and removal of the pressurized fluidsoccur, respectively;

(2) the maximum pressure level to which the heating gas is injected intothe oil shale; and

(3) the pressure ditferential producing transient fluid flowingconditions in the oil shale during the injection of heating gas into,and the removal of presssurized fluids from, the oil shale.

References Cited UNITED STATES PATENTS 2,862,557 12/1958 Van Utenhove eta1. 166-11 2,899,186 8/ 1959 Crawford.

3,004,595 10/ 1961 Crawford.

3,084,919 4/1963 Slater 16611 3,139,928 7/ 1964 'Broussard 166--40 X3,182,721 5/1965 Hardy.

3,232,345 2/1966 Trantham et al. 166-11 STEPHEN J. NOVOSAD, PrimaryExaminer.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,362,471 January 9, 1968 Marion L. Slusser et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 5, line 13, for "pressuree" read H pressure column 6, line 62,for "pressure" read pressures column 7 line 53 for "of" read or column12 line 21 for "presurized" read pressurized same column 12, TABLE I, inthe heading to the third column, lineS thereof, for 1 read F. -;column13, line 63, for "quanity" read quantity column 14, line 55, for"obtained" read obtain column 16, line 28, for "removed" read recoveredSigned and sealed this 25th day of February 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

